In order to make reliable estimates of the risks presented by environmental mutagens an intimate understanding of the mutagenic lesions, and the DNA repair processes that moderate them, is essential. The achievement of such detailed insights requires both quantitative and qualitative data on mutation at the molecular level. This project involves the production of mutational spectra over a range of doses in different repair backgrounds. The data produced will provide a basis for the identification of mutational lesions, and improve our understanding of the role of DNA repair in mutation avoidance and mutation fixation. Determination of mutational specificity is carried out in the lacI gene of E. coli. Using genetic tools, amber and ochre mutations are identified, mapped and characterized by suppression with known suppressors. Since the DNA sequence is known we then know the base substitution required to produce the mutation. In addition a rapid cloning and sequencing techniques has been developed allowing us to determine mutational spectra at the DNA sequence level.